Known injectors comprise an injector body, which defines a nozzle for injecting the fuel into the engine, and houses a metering valve activated by an electromagnetic actuator to open and close the nozzle. The valve comprises a control chamber communicating with a fuel inlet and defined by an end wall having a calibrated outlet hole; and a movable shutter, which is activated by the actuator to mate in fluid tight manner with the end wall and close the calibrated hole to vary the pressure in the control chamber.
More specifically, the shutter engages a conical seat defined by an end portion of the calibrated hole, and provides for fluid tight sealing along a circular contact line.
Known fuel injectors of the above type are unsatisfactory, not only on account of the difficulty and expense of machining the conical seat to the necessary roughness and tolerance values, but more importantly on account of the relatively severe wear to which the shutter and the end wall are subjected along the circular contact line where fluidtight sealing should be ensured. Such wear is substantially due to the relatively high operating speed of the shutter, which normally tends to exert severe, rapid closing forces along the circular contact line, thus resulting in impact which tends to cut into the conical seat.
To eliminate the latter drawback, injectors are known in which the end wall and the shutter mate in fluidtight manner along respective facing, parallel, complementary contact surfaces to close the calibrated hole.
Known solutions of the above type, however, call for relatively high lift of the shutter with respect to the end wall, and therefore relatively large, high-cost actuators requiring relatively high electric control currents. And despite this, wear along the contact surfaces is still relatively severe, by the high lift of the shutter still resulting in impact on the end wall.
The need for a relatively high lift is due to the formation, in use, of vortex regions in the fuel discharging from the calibrated hole, and therefore cavitation caused by the considerable difference in pressure between the calibrated hole and the outside. Which cavitation causes part of the fuel to pass from the liquid to the vapor phase, thus reducing fuel outflow from the calibrated hole, so that the discharge coefficients, and therefore the flow section between the end wall and the shutter, must be maintained high.